Valve announces plans to bring their Steam platform and Source engine to the Mac platform, confirming prettyobvious clues that this was in the works, saying Steam and Valve's library of games including Left 4 Dead 2, Team Fortress 2, Counter-Strike, Portal, and the Half-Life series will be available for Mac in April. Word is Portal 2 will be the first of their new titles to be simultaneously released on both platforms, and that developing in Source automates this: "Checking in code produces a PC build and Mac build at the same time, automatically, so the two platforms are perfectly in lock-step," said Josh Weier, Portal 2 Project Lead. "We're always playing a native version on the Mac right alongside the PC. This makes it very easy for us and for anyone using Source to do game development for the Mac."

They say they explored emulation, but decided to go with native versions: "We are treating the Mac as a tier-1 platform so all of our future games will release simultaneously on Windows, Mac, and the Xbox 360. Updates for the Mac will be available simultaneously with the Windows updates. Furthermore, Mac and Windows players will be part of the same multiplayer universe, sharing servers, lobbies, and so forth. We fully support a heterogeneous mix of servers and clients. The first Mac Steam client will be the new generation currently in beta testing on Windows." They are also adding a new feature, called Steam Play, which allows customers who purchase the product for the Mac or Windows to play on the other platform free of charge, and Jason Holtman, Director of Business Development at Valve, says: "For example, Steam Play, in combination with the Steam Cloud, allows a gamer playing on their work PC to go home and pick up playing the same game at the same point on their home Mac. We expect most developers and publishers to take advantage of Steam Play."

I have to guess MAC gamers (if there is such a thing)know what to expect if they purchase both PC and MAC games. If they only purchase MAC games, they certainly aren't going to see a long list

Initially, yes, but if they can write once and build on both platforms, then that list will grow - providing they can convince people to use Source.

Of course, I don't think this is the magic bullet - sometimes we end up doing things on one platform one way but find that when we try to do the same thing on a different platform, it doesn't perform or behave in exactly the same way.

I think this is great though - I've got a steam catalog full of games (almost all the Source-based titles) that I haven't touched in ages that I'd like to be able to play in OS X.